Programmed cell death, or apoptosis, is important for the development of most organs also for adult tissue homeostasis and remodeling. However, an inexorable loss of terminally differentiated heart muscle cells is a crucial causal factor for heart failure, the current leading cause of death in developed countries. Here, we demonstrate that HSG (also named mitofusin-2), a mitochondria protein, is a major determinant of oxidative stress-mediated cardiomyocyte apoptosis. Myocardial infarction profoundly elevates endogenous HSG expression and myocyte apoptosis in vivo via a reactive oxygen species (ROS)-dependent mechanism. Similarly, oxidative stress with H2O2 concurrently increases HSG expression and apoptosis in cultured rat cardiomyocytes. Furthermore, adenoviral gene transfer-mediated overexpression of HSG is sufficient to suppress both basal and serum-stimulated Akt activation, and triggers robust cardiomyocyte apoptosis. The HSG-induced apoptosis is fully abrogated by inhibition of caspase-9 but not caspase-8, and by expression of a constitutively active PI3K mutant to activate Akt or a mitochondrial antiapoptotic protein, Bcl-xL, indicating that HSG promotes cardiomyocytes apoptosis via inhibition of the PI3K-Akt cell survival signaling and the resultant activation of the primary mitochondrial apoptotic pathway. Importantly, siRNA-mediated HSG silencing protects cells against oxidative stress-induced apoptosis. The present results indicate that cardiac HSG functions as a major determinant of heart muscle cell apoptosis in response to ischemia and oxidative stress, via inhibiting the primary cell survival signal, Akt, resulting in activation of the mitochondrial cell death pathway. Importantly, increased cardiac HSG expression is both necessary and sufficient for oxidative stress-induced heart muscle cell apoptosis, suggesting that HSG deregulation may be a crucial pathogenic element and a promising therapeutic target for heart failure.